Metal carbide coatings on metal substrates

ABSTRACT

A METAL SUBSTRATE SELECTED FROM STEEL, NICKEL AND NICKELBASE ALLOYS HAVING A REFRACTORY METAL DIFFUSED INTO A SUBFACE REGION THEREOF IS PROVIDED WITH A COHERENT COAT OF A REFRACTORY METAL CARBIDE ADHERENTYL BONDED TO THE DIFFUSED SURFACE REGION OF THE SUBSTRATE.

Jan. '5, 1971 J. R. DARNELL ET L METAL CARBIDE COATINGS ON METALSUBSTRATES Original Filed Aug. 5, 1964 1' 5 Z 2 I \m CO :EH2 OR 22 l 5552 INERT GAS w VARIABLE ELECTRIC POWER H2 SUPPLY H2 OR OR INERT GASINERT GAS INVENTOR United States Patent 3,552,939 METAL CARBIDE COATINGS0N METAL SUBSTRATES James R. Darnell and Donald A. Tarver, Richardson,

Tex., assignors to Texas Instruments Incorporated,

Dallas, Tex., a corporation of Delaware Original application Aug. 5,1964, Ser. No. 387,725, now

Patent No. 3,368,914, dated Feb. 13, 1968. Divided and this applicationDec. 6, 1967, Ser. No. 721,886

Int. Cl. B321) /16 US. Cl. 29-195 16 Claims ABSTRACT OF THE DISCLOSURE Ametal substrate selected from steel, nickel and nickelbase alloys havinga refractory metal diffused into a surface region thereof is providedwith a coherent coat of a refractory metal carbide adherently bonded tothe diffused surface region of the substrate.

This application is a division of application Ser. No. 387,725, filedAug. 5, 1964, now US. Pat. 3,368,914, issued Feb. 13, 1968. I

The present invention relates to metal carbide products and moreparticularly, but not by way of limitation, relates to relatively thickmetal carbide coatings adherently bonded to relatively thick metalsubstrates, and more specifically, to tungsten carbide coatings bondedto steel substrates.

In copending US. application Ser. No. 387,613, filed by Donald A. Tarveron even date herewith, now U.S. Pat. 3,389,977, and entitled CVD Processfor Producing Tungsten Carbide and Article of Manufacture, and assignedto the assignee of the present invention, a process for producingtungsten carbide, molybdenum carbide, and other metal carbides isdisclosed and claimed. In general, the disclosed process comprisesreacting a metal halide with carbon monoxide, preferably in the presenceof hydrogen, to produce a coherent metal carbide. In particular, thedisclosed process involves the reaction of tungsten hexafiuoride andcarbon monoxide to produce tungsten carbide.

Tungsten carbide is particularly useful for tipping machine cuttingtools due to its extreme hardness but is too brittle to be useful forthis purpose unless backed by the tensile strength of a metal substratesuch as steel. The adherent bonding of tungsten carbide to a metalsubstrate, and in particular to steel, presents a considerable problemsince the coefficient of linear thermal expansion of carbon steel isalmost two and one-half times as great as that of tungsten carbide.Theerfore when the steel substrate is cooled after the tungsten carbidehas been deposited at high temperature, the tungsten carbide tends tospall off. As described in the above-referenced application, thetungsten carbide can be made readily adherent to a steel substrateprovided either the tungsten carbide coating or the steel substrate isrelatively thin because the thin member will deform sufficiently tocompensate for the difference in thermal contraction. It is alsodisclosed in the above-referenced copending application that thetungsten carbide can be better bonded to the steel substrate by a thinfilm of tungsten metal laid down by a chemical vapor deposition process,such as by the reduction of tungsten hexafiuoride in the presence ofhydrogen.

The present invention is concerned with adherently bonding a coat oftugsten carbide, or other metal carbide of substantial thickness, to asteel, or other metal substrate, also of substantial thickness, so thatthe resulting article will have a very hard surface, yet will haveconsiderable tensile strength. In accordance with the present 3,552,939Patented Jan. 5, 1971 invention, a metal carbide coating is adherentlyapplied to the surface of a metal substrate by first diffusing a secondmetal into the surface of the substrate so as to relax the thermalexpansion coefficient of the surface zone of the substrate. Then themetal carbide is deposited on the diffused surface of the substrate by achemical vapor deposition process. In general, it is preferable todiffuse the metal forming the carbide into the substrate. For example,in the situation where tungsten carbide is to be adherently bonded to asteel substrate, tungsten is preferably diffused into the steelsubstrate prior to the deposition of the tungsten carbide.

The tenacity of the bond between the metal carbide and the metalsubstrate may be substantially enhanced by also depositing a thin filmof a second metal over the surface of the diffused substrate. Again itis preferred that the metal film be the metal forming the carbide. Thustungsten is diffused into the surface of the steel substrate, then athin film of tungsten is deposited on the diffused surface. The tungstencarbide coat is then deposited on the tungsten film. It is alsopreferable to partially diffuse the tungsten metal film into the alreadydiffused surface of the substrate.

The process of this invention is generally useful for bonding tungstencarbide, molybdenum carbide, and other carbides to substantially anymetal substrate, and in particular for bonding tungsten carbide andmolybdenum carbide to iron and nickel-base alloys. In general, thebonding of a thick coat of tungsten carbide to a thick steel substraterepresents the most difficult task because the variance between thecoefficients of thermal expansions of these two materials is greaterthan between any other metal carbide and metal which might be ofinterest.

In accordance with a more specific aspect of the invention, tungsten isdiffused into the surface of a steel substrate by either a packdiffusion process or by partially diffusing a tungsten film applied tothe substrate by chemical vapor deposition. If the pack diffusionprocess is used, a film of tungsten is preferably deposited on thesubstrate surface by a chemical vapor deposition process and thenpartially diffused by raising the substrate to a temperature in excessof the deposition temperature for a considerable period of time. Thenfinally the tungsten carbide is deposited on the tungsten film over thesteel substrate by the reaction of tungsten hexafluoride and carbonmonoxide in the presence of hydrogen.

An important object of this invention is to provide a steel substratehaving an adherent coat of coherent tungsten carbide over the surfacethereof.

A further object of this invention is to provide a structure comprisingan adherently deposited metal carbide upon a metal substrate,particularly where both are so thick as not to elastically deform tocompensate for a mismatch in thermal expansivity.

Many additional objects and advantages of the present invention will beevident to those skilled in the art from the following detaileddescription and drawings, wherein:

FIG. 1 is a schematic diagram of a system which may be used to carry outthe process of the present invention;

FIG. 2 is a sectional view, somewhat schematic, of a portion of anarticle of manufacture constructed in accordance with the presentinvention; and

FIG. 3 is a sectional view, somewhat schematic, of a portion of anarticle of manufacture constructed in accordance with the presentinvention.

Referring now to FIG. 1, a system for carrying out the process of thepresent invention is indicated generally by the reference numeral 10.The system 10 is comprised of a controlled atmosphere reaction chamber12 which may be operated at atmospheric pressure in which a substrate 14to be coated may be supported. The substrate 14 may be heated by anysuitable means, such as by resistive heating from the variable electricpower supply 16. One conduit system for carrying out one specificprocess of the present invention is indicated generally by the referencenumeral 18 and is comprised of valved conduits 20, 22 and 24 which areconnected to a common inlet 26 to the chamber 12. The valved conduits20, 22 and 24 are connected to a source of hydrogen or an inert gas, asource of tungsten hexafluoride gas and a source of carbon monoxide gas,respectively. A conduit system for carrying out another specific processin accordance with the present invention is indicated generally by thereference numeral 28 and is comprised of valved conduits 30 and 32 whichare connected to sources of hydrogen or inert carrier gas and tungstenhexafiuoride gas, respectively. A third valved conduit 34 is connectedto a source of hydrogen or inert carrier gas and extends to the bottomof a container 36 containing particulate tungsten hexacarbonyl. Thetungsten hexacarbonyl is heated by a suitable means (not illustrated) toestablish the desired vapor pressure such that hydrogen passing throughthe container 36 will entrain the tungsten hexacarbonyl and be passedthrough the valved conduit 38 and mixed with the gases from the valvedconduits 30 and 32 prior to introduction through the inlet 40 to thereaction chamber 12. A valved outlet conduit 42 is provided forwithdrawing gases from the chamber 12 in order to establish a fiow ofreactants past the substrate 14.

In carrying out one specific embodiment of the invention, a steelsubstrate 14a is properly cleaned and is placed in the reaction chamber12 and connected to the source of electrical power 16 so as to beresistively heated. It is to be understood, however, that the substrate14 may be heated in any suitable manner, such as for example, byinductive or radiant heating techniques. Next the valved conduit 20 isopened so as to admit hydrogen to the chamber 12 and the valved conduit42 opened so that the chamber is purged of foreign gases. Next thesubstrate 14 is heated to a temperature greater than 400 C., andpreferably between about 600 C. and about 1,000 C., such as for example,800 C. The valved conduit 22 is then opened so as to mix tungstenhexafluoride with the hydrogen stream from the valved conduit 20'. Aflow of a mixture of hydrogen and tungsten hexafluoride is thenestablished past the heated substrate 14 so as to deposit a film oftungsten on the surface of the substrate by the hydrogen reduction ofthe tungsten hexafluoride to form elemental tungsten. The flow oftungsten hexafluoride is then stopped, and the substrate 14 raised to atemperature from about 1,000 C. to about 1,200 C. for a significantperiod of time, preferably for several hours, so that the tungsten metalwill partially diffuse into the surface of the steel substrate.

Next the substrate 14 is cooled to a temperature at which a coat ofcoherent tungsten carbide may be deposited as described in detail in theabove-referenced application. This entails admitting a mixture ofhydrogen, tungsten hexafluoride and carbon monoxide through the valvedconduits 20, 22 and 24, respectively. The mixture of gases is introducedto the chamber 12 and passed across the substrate 14 and withdrawnthrough the outlet conduit 42 at the desired flow rate. The temperatureof the substrate should be greater than about 400 C. and is preferablyin the range from about 600 C. to about 1,000 C. but it is believed thatthe process may go as high as the properties of the substrate willpermit. However, the higher the temperature the faster the depositionrate and the greater the tendency to produce a rough coating havingrelatively long spikes protruding therefrom. In the alternative, theconduit system 28 may be used to deposit the tungsten carbide in whichcase the valved conduits 30, 32, 34 and 38 are opened so as to admit areactant mixture of hydrogen, tungsten hexafluoride, and the vapors oftungsten hexacarbonyl. The parameters of temperature and flow aresubstantially the same as the corresponding parameters when using carbonmonoxide in the system 1 8. The resulting coherent tungsten carbide willbe adherently 4 bonded to the surface of the steel substrate into whichtungsten has previously been diffused.

An alternative process for diffusing tungsten or other metal into thesurface of the steel substrate may be employed without departing fromthe broad aspects of the present invention. Specifically, tungsten maybe diffused into the steel substrate 14 prior to placement of thesubstrate in the reaction chamber 12 by a pack diffusion process. Inthis process particulate tungsten is mixed with a halide and hydrogencompound, such as for example, an ammonium halide, diluted with an inertingredient such as alumina. The substrate is packed in this mixture in areceptacle and heated to an elevated temperature of from about l,000 C.to about 1,200" C. for several hours until the desired degree ofdiffusion is obtained.

In general, the degree of diffusion desired by either diffusion processis from zero at the inner face of the diffusion front a few mils deep inthe substrate to a concentration as high as 50% at the surface of thesubstrate. The result is a steel substrate with a tungsten-rich case.The tungsten is believed to displace the molecules of iron and assumethe positions of the iron molecules in the steel crystal structure.After the substrate 14 has been prepared by the diffusion of thetungsten, it is then placed in the reaction chamber 12 and a coat oftungsten carbide deposited over the diffused surface using either of theprocesses described briefly above and in greater detail in theabove-referenced application, that is by the reaction of tungstenhexafluoride and carbon monoxide in the presence of hydrogen.

The tenacity with which the tungsten carbide may be adherently bonded toa steel substrate may be materially increased by using the followingalternative process. First tungsten is diffused into the substrate 14using either the pack diffusion process or the chemical vapor depositionprocess, or any other suitable diffusion technique so asto produce atungsten-rich surface region around the steel substrate as previouslydescribed. Then a thin film of tungsten is deposited on the diffusedsurface of the steel sub strate by chemical vapor deposition or othertechnique. Finally the coat of tungsten carbide is deposited on thetungsten film by the reaction of tungsten hexafluoride and carbonmonoxide in the presence of hydrogen.

In this case the entire process may be carried out within the reactionchamber without disturbing the substrate 14. The substrate 14 is placedin the reaction chamber 12, the chamber purged with hydrogen, and thesubstrate 14 raised to a temperature in the range from about 600 C. toabout 800 C. and a mixture of hydro- .gen and tungsten hexafluoridepassed through the chamber to deposit a film of tungsten on the surfaceof the substrate. The substrate is then heated to a temperature fromabout 1,000 C. to about 1,200 C. for a substantial period of time,usually several hours to partially diffuse the tungsten into the surfaceof the steel substrate. The'substrate is then cooled to depositiontemperatures greater than 400 C. and preferably between about 600 C. andabout 1,000 C. and a mixture of hydrogen, tungsten hexafluoride andcarbon monoxide passed through the reaction chamber until a coating oftungsten carbide of the desired thickness is deposited on the surface ofthe tungsten film on the surface of the steel substrate 14.

Although the specific processes of the present invention have all beendescribed with regard to a steel substrate, a tungsten metal diffusion,a tungsten metal interlayer and tungsten carbide final coating, it willbe appreciated that in its broader aspects, the process also applies toother metal substrates, to other metal diffusion materials, to othermetal films on the substrate, and other metal carbide coatings. Ingeneral, adherently bonding relatively thick tungsten carbide to thesurface of a steel substrate represents the most difiicult problem to beencountered due to the considerable difference between the coefiicientsof thermal expansions of the two materials. Tungsten carbide on steelalso has the most important immediate commercial implications, and forthis reason, a major portion of our efforts have been directed towardachieving a tungsten carbide-to-steel bond. The adherence of other metalcarbides to other metal substrates, or the adherence of tungstencarbides to other metal substrates, or the adherence of other metalcarbides to steel substrates all present less difficult tasks andaccordingly considerable success may be expected in these areas by usingthe above-described processes. The process has been successfully used toproduce a molybdenum carbide coat adherently bonded to a steelsubstrate. The process is further particularly applicable to bondingtungsten carbide coatings to nickel-base alloys. In general, the termsteel as used herein includes almost all iron-base alloys. The metalused for the intermediate metal film and diffused regions may betungsten, molybdenum, chromium, niobium and tantalum, as well as mostother refractory metals which have other physical properties compatiblewith the particular use intended for the resulting article and with thetemperatures involved in the process.

The articles of manufacture resulting from the above process constitutean important aspect of the present invention. One such article isindicated .generally by the reference numeral 50 in FIG. 2 and iscomprised of a metal substrate 14a having a second metal diffused intothe surface region 52 of the substrate to relax the coefficient ofthermal expansion of the substrate metal. A coat of coherent metalcarbide 54 is adherently bonded to the diffused surface of thesubstrate. In particular, the article 50 is comprised of a steelsubstrate 14a, a tungstendiffused region 52 and a substantially coherenttungsten carbide coating 54 having no significant metal phase and ahardness which exceeds commercially available tungsten carbide.

Another article constructed in accordance with the present invention isindicated generally by the reference numeral 60 in FIG. 3. The article60 is comprised of a metal substrate 14b having a diffused surfaceregion 62 rich in a second metal as a result of the diffusion of themetal into the surface region. A thin metallic layer 64 is adherentlybonded to the diffused surface of the substrate 14b, and a coat ofcoherent, substantially pure metal carbide 66 is adherently bonded tothe metal layer 64. In particular, the substrate 14b is preferably steelor iron base alloy, the diffused metal in the region 62 is preferablytungsten, the metal layer 64 is preferably tungsten, and the metalcarbide coating 66 is preferably tungsten carbide. However, thesubstrates 14a and 14b may be nickel or nickel-base alloys orsubstantially any other metal having the thermal and strengthcharacteristics which might be of interest for a particular applicationof the metal carbide coating. In particular, the metal in the diffusedregions 52 and 62 and the metal forming the layer 64 may be anycombination of tungsten, molybdenum, chromium, niobium or tantalum.

Having thus described several preferred embodiments of the presentinvention, it will be evident to those skilled in the art that a highlyuseful process has been described for producing a novel and usefularticle having a coat of metal carbide adherently bonded to the surfaceof a metal body. The article has the combined properties of the twomaterials, namely the hardness of the metal carbide and the tensilestrength of the metal body. The uses to which the articles may be placedare widely known and include, for example, machine tools and drill-proofarmor for safes and the like.

Although several preferred embodiments of the invention have beendescribed in detail, it is to be understood that various changes,substitutions and alterations can be made therein without departing fromthe spirit and scope of the invention as defined by the appended claims.

What is claimed is:

1. An article of manufacture comprising:

a metal substrate selected from steel, nickel and nickelbase alloyshaving a refractory metal diffused into a surface region of the metalsubstrate, and

a coherent coat of a refractory metal carbide adherently bonded to thediffused surface region of the metal substrate.

2. An article of manufacture as defined in claim 1 wherein the metalcarbide is tungsten carbide.

3. An article of manufacture as defined in claim 1 wherein the metalsubstrate is steel.

4. An article of manufacture as defined in claim 1 wherein the secondmetal is tungsten.

5. An article of manufacture as defined in claim 1 wherein the secondmetal and the metal of the metal carbide are the same metal.

6. An article of manufacture as defined in claim 5 wherein the metal istungsten.

7. An article of manufacture as defined in claim 5 wherein the metal ismolybdenum.

8. An article of manufacture comprising:

a metal substrate selected from steel, nickel and nickelbase alloyshaving a second metal selected from tungsten, molybdenum, chromium,niobium and tantalum diffused into the surface region of the metalsubstrate,

a metal film selected from tungsten, molybdenum, chromium, niobium andtantalum adherently bonded to the diffused surface region, and

a coherent coat of a metal carbide selected from tungsten molybdenum,chromium, niobium and tantalum carbides adherently bonded to the metalfilm.

9. An article of manufacture as defined in claim 8 wherein the secondmetal diffused into the substrate is tungsten.

10. An article of manufacture as defined in claim 8 wherein the metalfilm is tungsten.

11. An article of manufacture as defined in claim 8 wherein the metalcarbide is tungsten carbide.

12. An article of manufacture as defined in claim 8 wherein thesubstrate is steel.

13. An article of manufacture as defined in claim 8 wherein thesubstrate is a nickel-base alloy.

14. An article of manufacture as defined in claim 8 wherein the secondmetal, the metal film and the metal of the carbide are the same metal.

15. An article of manufacture as defined in claim 14 wherein the samemetal is tungsten.

16. An article of manufacture as defined in claim 14 wherein the samemetal is molybdenum.

References Cited UNITED STATES PATENTS 3,023,490 3/1962 Dawson 29l95X3,139,329 6/1964 Zeller 29195 3,260,579 7/1966 Scales et al 291953,326,714 6/1967 Rath 29l95X L. DEWAYNE RUTLEDGE, Primary Examiner E. L.WEISE, Assistant Examiner

